If I was/were rich ... are identical in meaning. They are 2nd conditional. The 'correct' form is the subjunctive 'were.' However, so many people use the past simple 'was' that it is considered acceptable now. BUT I WOULD USE THE SUBJUNCTIVE IF YOU TAKING AN EXAM. It's a safer choice. It is the 'correct' form. When I was a girl it was considered very wrong to use 'was.' Times have changed, and it is not considered so wrong now.